Drilling operations in gas and oil exploration involve driving a drill bit into the ground to create a borehole (i.e., a wellbore) from which oil and/or gas are extracted. The drill bit is installed at the distal end of a drill string, which extends from a derrick on the surface into the borehole. The drill string is formed by connected a series of drill pipes together. A bottom hole assembly (BHA) is installed proximately above the drill bit in the drill string.
The BHA contains instruments that collect and/or transmits information regarding the drilling tools, wellbore conditions, earth formation, etc. to the surface. The information is used to determine drilling conditions such as, drift of the drill bit, inclination and azimuth, which in turn are used to calculate the trajectory of the borehole. Real-time data are important in monitoring and controlling the drilling operation, either by automatic control or operator intervention.
Technology for transmitting information within a wellbore, known as telemetry technology, is used to transmit the information from the BHA to the surface for further analysis. One of the known telemetry methods is mud pulse telemetry, which uses drilling mud to carry information from downhole to the surface. Drilling mud, aka drilling fluid, is pumped by a mud pump from surface down the wellbore through the conduit inside the drill string and circulates back to the surface through the annular space between the drill string and the wellbore.
The flow of the drilling mud through the drill string may be modulated (i.e., encoded) by a mud pulser to cause pressure and/or flow rate variations. The pressure or flow rate variations are captured by a corresponding sensor at or near the surface and decoded using a decoding software to recover the downhole information. The mud pulser can be a part of the BHA in a system using mud pulse telemetry.
Specific designs of a mud pulser may vary but the basic principle is that the mud pulser generates pressure pulses by constricting a flow path in the mud flow in the borehole. The mud flow is constricted or released in the drill string with according to a specific timing sequence to encode data in the modulated pressure pulses in the mud flow. The modulated pressure pulses propagate through the mud flow to the surface, which are detected and decoded at the surface to retrieve the original data.
Mud pumps, which provide the motive force to the mud flow, are large positive displacement pumps that drive the mud flow by moving a piston back and forth within a cylinder while simultaneously opening and closing intake and exhaust valves. A typical mud pump has three pistons attached to a common drive shaft. These pistons are one hundred and twenty degrees out of phase with one another to minimize pressure variations. A dampener is used to reduce the pulsation in the mud flow.
In addition to mud pulse telemetry, wired drill pipe telemetry is also frequently used in drilling operations. In wired drill pipe telemetry, the drill pipes in a drill string have communication cable embedded in the drill pipe wall. When the drill pipes are connected together, sections of communication cable form a continuous communication cable from the BHA to the surface along the drill string. The advantage of the wired telemetry is that the data transmission through the cable is bidirectional and is much faster than that of mud pulse telemetry. However, connecting two sections of communication cable at the joint between two drill pipes requires sophisticated and expensive coupling devices. When drilling a deep well, many of such joints are needed. Breakage of the communicate cable at any of the joints would disable the telemetry, which requires expensive repairs. For this and other reasons, mud pulse telemetry is still widely used in drilling operations nowadays.
Differing from bidirectional wired telemetry, the mud pulse telemetry normally telemeters data from downhole to the surface. There is a need for methods and systems that telemeter signals from the surface to tools in the borehole downhole.